1. Field of the Invention
The present invention relates to a display system suitable for use with a digital computer, and more particularly to a display system with a data structure which is suited to be microprogram-controlled.
2. Description of the Prior Art
As disclosed in "A high-resolution raster scan display" by Jean Clande Roy. Hewlett-Pakard Journal Vol. 26, No. 10, pp 11 - 15, for example, a common construction of the display system comprises a data control unit for controlling the exchange of data with an external data source, such as a digital computer or a keyboard, a refresh memory for storing display data, a timing control unit for controlling the input/output timing as well as the display timing, a video control unit for generating a video signal for display based on the display data and the display timing, and a viewer for displaying the video signal as a visible image.
It is the control function of the data control unit and the video control unit which has the primary influence on the function of the entire display system. With a recent trend of increased intelligence included in the data processing and the complexity of the display function, the control function of those units is becoming more and more important with the attendant increase in the amount of hardware in the control units. As an approach to overcome the above problem it has been proposed to enhance the flexibility of processing and reduce the amount of hardware by a microprogram control which uses a recently developed one-chip CPU (micro-processor), as is disclosed in "Micro-processors in CRT terminal applications: Hardware/Software" by Michael T. Gray. Computor Oct. 1975 pp 53 - 59, for example.
A data structure for conventional display systems employing a microprogram control has been constructed such that the data control unit, the refresh memory unit and the timing control unit are connected to a common address bus and a common data bus, and the data control unit has been usually constructed mainly by a microprocessor and a microprogram memory. The refresh memory usually stores a screen of data and the exchange of data with an external data source is carried out under the control of the micro-processor and the microprogram memory.
For display, the data at a selected address are is read out in synchronism with the scan rate of the viewer under the control of the timing control unit. In order to facilitate the efficient use of the micro-processor, a line buffer is usually provided so that a line of display data is transferred from the refresh memory to the line buffer in a direct memory access (DMA) mode during the line spacing and the input/output control for the refresh memory is carried out while the line of data of the line buffer is being displayed.
In the display system of the type described above, if the micro-processor is byte-oriented (e.g. 8-bit byte), the format for the display data must be also byte-oriented because of the common data bus. Accordingly, the freedom of the display and the restriction the entry region is low and the versatility of the function is poor. Furthermore, while the data is being transferred from the refresh memory to the line buffer under the DMA mode, the micro-processor cannot access the refresh memory, resulting in a low efficiency of data processing. In addition, additional hardware for the line buffer and a DMA timing generation circuit are necessary.